DC-DC (direct current) converters convert DC power at one voltage to DC power at a different voltage. DC-DC converters commonly include a feedback amplifier, also called a compensation filter, where “compensation” means that the feedback network gain and phase as a function of frequency ensure that the overall system with feedback is stable. The feedback signal may be voltage or current, and there may be multiple feedback loops.
FIG. 1 illustrates an example DC-DC converter 100. The DC-DC converter illustrated is a switching step-down converter (the output voltage is less than the input voltage). The particular topology is called a Buck converter. Buck converters are particularly suitable for power supplies for battery powered low-voltage microprocessor and microcontroller applications. The circuit illustrated in FIG. 1 converts DC power from voltage VIN to DC power for a load R at voltage VOUT. When switch SW1 is closed, a first end of inductor L is at VIN, and when switch SW2 is closed, the first end of inductor L is at ground. The inductor L stores energy and prevents instantaneous current change. A capacitor C stores energy and prevents instantaneous voltage change, thereby reducing output voltage ripple. In the example circuit 100, output voltage VOUT is compared to a reference voltage VREF by a feedback compensation filter 102. A ramp generator 104 is driven by a clock signal CLK. A comparator 106 compares the output of the compensation filter 102 to the ramp signal. A driver circuit 108 closes SW1 at the start of each clock cycle, and the point at which the comparator 106 switches determines the width of a pulse driving SW1. During the remainder of the clock cycle, the driver 108 closes switch SW2.
In general, there is a need to extend the high frequency response of a DC-DC converter system while maintaining stability. This is especially important, for example, in applications where significant loads such as microprocessors and microcontrollers may be frequently switched in and out of standby mode to reduce power. There are three commonly implemented compensation filters for Buck converters, which are classified according to their bandwidth and their gain and phase response as a function of frequency. A Type-I filter is essentially an integrator, providing good DC voltage or DC current regulation at a relatively low bandwidth. A Type-II filter extends the bandwidth and provides a phase shift to ensure stability at the extended bandwidth. A Type-III filter further extends the bandwidth and provides additional phase shift.
A compensation filter may be implemented as an analog circuit (with external discrete analog components or with integrated analog components). Alternatively, a compensation filter may be implemented as an all-digital circuit. For analog implementations, each higher filter type requires additional passive components (resistors and capacitors). Typically, for analog implementations, many of the passive components are external discrete components, which require space and the additional cost of mounting. Alternatively, the passive analog components may be integrated, but this usually requires significant integrated circuit space and expense. In addition, time constants for filters with integrated analog components may vary by +/−40%, requiring extensive trimming, which increases cost and processing time. In addition, all of the component values need to be variable because all the filter time constants need to track the switching frequency of the Buck converter. For an all-digital implementation, the analog output signal may be converted to digital signals with an analog-to-digital converter, the resulting digital signals may be processed by a digital-signal-processor, and the resulting digital signals may be converted to analog signals by a digital-to-analog converter. Typically, an all-digital filter circuit is physically smaller than an analog circuit, but consumes more power.
There is an ongoing need for improved compensation filters.